The Trim By Surfaces command is available regardless of current selection. This command is similar to the Create, Intersect command in that it works on two groups of entities, Group A and Group B, which can each be trimmed by the other.

trim by Surfaces
Use the Trim By Surfaces command to trim surfaces, quilts and models with each other.

Upon entering the command, any selected database surfaces, quilts or models will be added to Group A. Entity Selection will be defaulted to Group B selection. Otherwise, if no trimming database entities are selected prior to entering the command, Entity Selection will default to Group A. After both groups have been defined, the Imprint command becomes available. Click Imprint to have the groups intersected against each other for trimming locations. The table updates to show the number of entities in each group as they are selected as well as the number of quilts defined after the imprint process. In the Tolerance frame, adjust the Intersection Tolerance if necessary to obtain expected trimming intersections. Press the Reselect Entities button to clear trim results with the previous selections of Group A and Group B maintained.

In the Advanced frame, choose which of the groups is to be trimmed by toggling either or both of Trim A with B and Trim B with A. Also choose whether to Keep Inside quilts, to Keep Outside quilts, or both. The orientation of the inside or outside is defined by surface normals. That is, the outside of a surface refers to the directions aligned with the normals. Note that the Orient command can be used to change the orientation of database surfaces as necessary to facilitate use of the Keep Inside and Keep Outside options. Click OK to save all of the designated trimmed quilts and exit the command, or click Apply and Select to interactively select the desired trimmed portions in the Display window. Click Cancel to exit the command without saving any changes.